RFC 3723 Securing Block Storage Protocols over IP April 20041. Introduction
This specification discusses use of the IPsec protocol suite for
protecting block storage protocols over IP networks (including iSCSI,
iFCP and FCIP), as well as storage discovery protocols (iSNS and
SLPv2).
1.1. iSCSI Overview
iSCSI, described in [RFC3720], is a connection-oriented
command/response protocol that runs over TCP, and is used to access
disk, tape and other devices. iSCSI is a client-server protocol in
which clients (initiators) open connections to servers (targets) and
perform an iSCSI login.
This document uses the SCSI terms initiator and target for clarity
and to avoid the common assumption that clients have considerably
less computational and memory resources than servers; the reverse is
often the case for SCSI, as targets are commonly dedicated devices of
some form.
The iSCSI protocol has a text based negotiation mechanism as part of
its initial (login) procedure. The mechanism is extensible in what
can be negotiated (new text keys and values can be defined) and also
in the number of negotiation rounds (e.g., to accommodate
functionality such as challenge-response authentication).
After a successful login, the iSCSI initiator may issue SCSI commands
for execution by the iSCSI target, which returns a status response
for each command, over the same connection. A single connection is
used for both command/status messages as well as transfer of data
and/or optional command parameters. An iSCSI session may have
multiple connections, but a separate login is performed on each. The
iSCSI session terminates when its last connection is closed.
iSCSI initiators and targets are application layer entities that are
independent of TCP ports and IP addresses. Initiators and targets
have names whose syntax is defined in [RFC3721]. iSCSI sessions
between a given initiator and target are run over one or more TCP
connections between those entities. That is, the login process
establishes an association between an iSCSI Session and the TCP
connection(s) over which iSCSI PDUs will be carried.
While the iSCSI login may include mutual authentication of the iSCSI
endpoints and negotiation of session parameters, iSCSI does not
define its own per-packet authentication, integrity, confidentiality
or replay protection mechanisms. Rather, it relies upon the IPsec
protocol suite to provide per-packet data confidentiality and
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RFC 3723 Securing Block Storage Protocols over IP April 2004
integrity and authentication services, with IKE as the key management
protocol. iSCSI uses TCP to provide congestion control, error
detection and error recovery.
1.2. iFCP Overview
iFCP, defined in [iFCP], is a gateway-to-gateway protocol, which
provides transport services to Fibre Channel devices over a TCP/IP
network. iFCP allows interconnection and networking of existing
Fibre Channel devices at wire speeds over an IP network. iFCP
implementations emulate fabric services in order to improve fault
tolerance and scalability by fully leveraging IP technology. Each
TCP connection is used to support storage traffic between a unique
pair of Fibre Channel N_PORTs.
iFCP does not have a native, in-band security mechanism. Rather, it
relies upon the IPsec protocol suite to provide data confidentiality
and authentication services, and IKE as the key management protocol.
iFCP uses TCP to provide congestion control, error detection and
error recovery.
1.3. FCIP Overview
FCIP, defined in [FCIP], is a pure FC encapsulation protocol that
transports FC frames. Current specification work intends this for
interconnection of Fibre Channel switches over TCP/IP networks, but
the protocol is not inherently limited to connecting FC switches.
FCIP differs from iFCP in that no interception or emulation of fabric
services is involved. One or more TCP connections are bound to an
FCIP Link, which is used to realize Inter-Switch Links (ISLs) between
pairs of Fibre Channel entities. FCIP Frame Encapsulation is
described in [RFC3643].
FCIP does not have a native, in-band security mechanism. Rather, it
relies upon the IPsec protocol suite to provide data confidentiality
and authentication services, and IKE as the key management protocol.
FCIP uses TCP to provide congestion control, error detection and
error recovery.
1.4. IPsec Overview
IPsec is a protocol suite which is used to secure communication at
the network layer between two peers. The IPsec protocol suite is
specified within the IP Security Architecture [RFC2401], IKE
[RFC2409][RFC2412], IPsec Authentication Header (AH) [RFC2402] and
IPsec Encapsulating Security Payload (ESP) [RFC2406] documents. IKE
is the key management protocol while AH and ESP are used to protect
IP traffic.
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An IPsec SA is a one-way security association, uniquely identified by
the 3-tuple: Security Parameter Index (SPI), protocol (ESP) and
destination IP. The parameters for an IPsec security association are
typically established by a key management protocol. These include
the encapsulation mode, encapsulation type, session keys and SPI
values.
IKE is a two phase negotiation protocol based on the modular exchange
of messages defined by ISAKMP [RFC2408],and the IP Security Domain of
Interpretation (DOI) [RFC2407]. IKE has two phases, and accomplishes
the following functions:
[1] Protected cipher suite and options negotiation - using keyed
MACs and encryption and anti-replay mechanisms
[2] Master key generation - such as via MODP Diffie-Hellman
calculations
[3] Authentication of end-points
[4] IPsec SA management (selector negotiation, options negotiation,
create, delete, and rekeying)
Items 1 through 3 are accomplished in IKE Phase 1, while item 4 is
handled in IKE Phase 2.
An IKE Phase 2 negotiation is performed to establish both an inbound
and an outbound IPsec SA. The traffic to be protected by an IPsec SA
is determined by a selector which has been proposed by the IKE
initiator and accepted by the IKE Responder. In IPsec transport
mode, the IPsec SA selector can be a "filter" or traffic classifier,
defined as the 5-tuple: <Source IP address, Destination IP address,
transport protocol (UDP/SCTP/TCP), Source port, Destination port>.
The successful establishment of a IKE Phase-2 SA results in the
creation of two uni-directional IPsec SAs fully qualified by the
tuple <Protocol (ESP/AH), destination address, SPI>.
The session keys for each IPsec SA are derived from a master key,
typically via a MODP Diffie-Hellman computation. Rekeying of an
existing IPsec SA pair is accomplished by creating two new IPsec SAs,
making them active, and then optionally deleting the older IPsec SA
pair. Typically the new outbound SA is used immediately, and the old
inbound SA is left active to receive packets for some locally defined
time, perhaps 30 seconds or 1 minute.
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RFC 3723 Securing Block Storage Protocols over IP April 20041.5. Terminology
Fibre Channel
Fibre Channel (FC) is a gigabit speed networking technology
primarily used to implement Storage Area Networks (SANs), although
it also may be used to transport other frames types as well,
including IP. FC is standardized under American National Standard
for Information Systems of the InterNational Committee for
Informational Technology Standards (ANSI-INCITS) in its T11
technical committee.
FCIP
Fibre Channel over IP (FCIP) is a protocol for interconnecting
Fibre Channel islands over IP Networks so as to form a unified SAN
in a single Fibre Channel fabric. The principal FCIP interface
point to the IP Network is the FCIP Entity. The FCIP Link
represents one or more TCP connections that exist between a pair
of FCIP Entities.
HBA
Host Bus Adapter (HBA) is a generic term for a SCSI interface to
other device(s); it's roughly analogous to the term Network
Interface Card (NIC) for a TCP/IP network interface, except that
HBAs generally have on-board SCSI implementations, whereas most
NICs do not implement TCP, UDP, or IP.
iFCP
iFCP is a gateway-to-gateway protocol, which provides Fibre
Channel fabric services to Fibre Channel devices over a TCP/IP
network.
IP block storage protocol
Where used within this document, the term "IP block storage
protocol" applies to all block storage protocols running over IP,
including iSCSI, iFCP and FCIP.
iSCSI
iSCSI is a client-server protocol in which clients (initiators)
open connections to servers (targets).
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iSNS
The Internet Storage Name Server (iSNS) protocol provides for
discovery and management of iSCSI and Fibre Channel (FCP) storage
devices. iSNS applications store iSCSI and FC device attributes
and monitor their availability and reachability, providing a
consolidated information repository for an integrated IP block
storage network. iFCP requires iSNS for discovery and management,
while iSCSI may use iSNS for discovery, and FCIP does not use
iSNS.
initiator
The iSCSI initiator connects to the target on well-known TCP port
3260. The iSCSI initiator then issues SCSI commands for execution
by the iSCSI target.
target
The iSCSI target listens on a well-known TCP port for incoming
connections, and returns a status response for each command
issued by the iSCSI initiator, over the same connection.
1.6. Requirements Language
In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL",
"RECOMMENDED", "SHALL", "SHALL NOT", "SHOULD", and "SHOULD NOT", are
to be interpreted as described in [RFC2119].
Note that requirements specified in this document apply only to use
of IPsec and IKE with IP block storage protocols. Thus, these
requirements do not apply to IPsec implementations in general.
Implementation requirements language should therefore be assumed to
relate to the availability of features for use with IP block storage
security only.
Although the security requirements in this document are already
incorporated into the iSCSI [RFC3720], iFCP [iFCP] and FCIP [FCIP]
standards track documents, they are reproduced here for convenience.
In the event of a discrepancy, the individual protocol standards
track documents take precedence.
2. Block Storage Protocol Security2.1. Security Requirements
IP Block storage protocols such as iSCSI, iFCP and FCIP are used to
transmit SCSI commands over IP networks. Therefore, both the control
and data packets of these IP block storage protocols are vulnerable
to attack. Examples of attacks include:
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[1] An adversary may attempt to acquire confidential data and
identities by snooping data packets.
[2] An adversary may attempt to modify packets containing data and
control messages.
[3] An adversary may attempt to inject packets into an IP block
storage connection.
[4] An adversary may attempt to hijack TCP connection(s)
corresponding to an IP block storage session.
[5] An adversary may launch denial of service attacks against IP
block storage devices such as by sending a TCP reset.
[6] An adversary may attempt to disrupt security negotiation
process, in order to weaken the authentication, or gain access
to user passwords. This includes disruption of application-
layer authentication negotiations such as iSCSI Login.
[7] An adversary may attempt to impersonate a legitimate IP block
storage entity.
[8] An adversary may launch a variety of attacks (packet
modification or injection, denial of service) against the
discovery (SLPv2 [RFC2608]) or discovery and management (iSNS
[iSNS]) process. iSCSI can use SLPv2 or iSNS. FCIP only uses
SLPv2, and iFCP only uses iSNS.
Since iFCP and FCIP devices are the last line of defense for a whole
Fibre Channel island, the above attacks, if successful, could
compromise the security of all the Fibre Channel hosts behind the
devices.
To address the above threats, IP block storage security protocols
must support confidentiality, data origin authentication, integrity,
and replay protection on a per-packet basis. Confidentiality
services are important since IP block storage traffic may traverse
insecure public networks. The IP block storage security protocols
must support perfect forward secrecy in the rekeying process.
Bi-directional authentication of the communication endpoints MUST be
provided. There is no requirement that the identities used in
authentication be kept confidential (e.g., from a passive
eavesdropper).
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For a security protocol to be useful, CPU overhead and hardware
availability must not preclude implementation at 1 Gbps today.
Implementation feasibility at 10 Gbps is highly desirable, but may
not be demonstrable at this time. These performance levels apply to
aggregate throughput, and include all TCP connections used between IP
block storage endpoints. IP block storage communications typically
involve multiple TCP connections. Performance issues are discussed
further in Appendix B.
Enterprise data center networks are considered mission-critical
facilities that must be isolated and protected from possible security
threats. Such networks are often protected by security gateways,
which at a minimum provide a shield against denial of service
attacks. The IP block storage security architecture should be able
to leverage the protective services of the existing security
infrastructure, including firewall protection, NAT and NAPT services,
and VPN services available on existing security gateways.
When iFCP or FCIP devices are deployed within enterprise networks, IP
addresses will be typically be statically assigned as is the case
with most routers and switches. Consequently, support for dynamic IP
address assignment, as described in [RFC3456], will typically not be
required, although it cannot be ruled out. Such facilities will also
be relevant to iSCSI hosts whose addresses are dynamically assigned.
As a result, the IP block storage security protocols must not
introduce additional security vulnerabilities where dynamic address
assignment is supported.
While IP block storage security is mandatory to implement, it is not
mandatory to use. The security services used depend on the
configuration and security policies put in place. For example,
configuration will influence the authentication algorithm negotiated
within iSCSI Login, as well as the security services
(confidentiality, data origin authentication, integrity, replay
protection) and transforms negotiated when IPsec is used to protect
IP block storage protocols such as iSCSI, iFCP and FCIP.
FCIP implementations may allow enabling and disabling security
mechanisms at the granularity of an FCIP Link. For iFCP, the
granularity corresponds to an iFCP Portal. For iSCSI, the
granularity of control is typically that of an iSCSI session,
although it is possible to exert control down to the granularity of
the destination IP address and TCP port.
Note that with IPsec, security services are negotiated at the
granularity of an IPsec SA, so that IP block storage connections
requiring a set of security services different from those negotiated
with existing IPsec SAs will need to negotiate a new IPsec SA.
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Separate IPsec SAs are also advisable where quality of service
considerations dictate different handling of IP block storage
connections. Attempting to apply different quality of service to
connections handled by the same IPsec SA can result in reordering,
and falling outside the replay window. For a discussion of the
issues, see [RFC2983].
IP block storage protocols can be expected to carry sensitive data
and provide access to systems and data that require protection
against security threats. SCSI and Fibre Channel currently contain
little in the way of security mechanisms, and rely on physical
security, administrative security, and correct configuration of the
communication medium and systems/devices attached to it for their
security properties.
For most IP networks, it is inappropriate to assume physical
security, administrative security, and correct configuration of the
network and all attached nodes (a physically isolated network in a
test lab may be an exception). Therefore, authentication SHOULD be
used by IP block storage protocols (e.g., iSCSI SHOULD use one of its
in-band authentication mechanisms or the authentication provided by
IKE) in order to provide a minimal assurance that connections have
initially been opened with the intended counterpart.
iSNS, described in [iSNS], is required in all iFCP deployments.
iSCSI may use iSNS for discovery, and FCIP does not use iSNS. iSNS
applications store iSCSI and FC device attributes and monitor their
availability and reachability, providing a consolidated information
repository for an integrated IP block storage network. The iSNS
specification defines mechanisms to secure communication between an
iSNS server and its clients.
2.2. Resource Constraints
Resource constraints and performance requirements for iSCSI are
discussed in [RFC3347] Section 3.2. iFCP and FCIP devices will
typically be embedded systems deployed on racks in air-conditioned
data center facilities. Such embedded systems may include hardware
chipsets to provide data encryption, authentication, and integrity
processing. Therefore, memory and CPU resources are generally not a
constraining factor.
iSCSI will be implemented on a variety of systems ranging from large
servers running general purpose operating systems to embedded host
bus adapters (HBAs). In general, a host bus adapter is the most
constrained iSCSI implementation environment, although an HBA may
draw upon the resources of the system to which it is attached in some
cases (e.g., authentication computations required for connection
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setup). More resources should be available to iSCSI implementations
for embedded and general purpose operating systems. The following
guidelines indicate the approximate level of resources that
authentication, keying, and rekeying functionality can reasonably
expect to draw upon:
- Low power processors with small word size are generally not used,
as power is usually not a constraining factor, with the possible
exception of HBAs, which can draw upon the computational resources
of the system into which they are inserted. Computational
horsepower should be available to perform a reasonable amount of
exponentiation as part of authentication and key derivation for
connection setup. The same is true of rekeying, although the
ability to avoid exponentiation for rekeying may be desirable (but
is not an absolute requirement).
- RAM and/or flash resources tend to be constrained in embedded
implementations. 8-10 MB of code and data for authentication,
keying, and rekeying is clearly excessive, 800-1000 KB is clearly
larger than desirable, but tolerable if there is no other
alternative and 80-100 KB should be acceptable. These sizes are
intended as rough order of magnitude guidance, and should not be
taken as hard targets or limits (e.g., smaller code sizes are
always better). Software implementations for general purpose
operating systems may have more leeway.
The primary resource concern for implementation of authentication and
keying mechanisms is code size, as iSCSI assumes that the
computational horsepower to do exponentiations will be available.
There is no dominant iSCSI usage scenario - the scenarios range from
a single connection constrained only by media bandwidth to hundreds
of initiator connections to a single target or communication
endpoint. SCSI sessions and hence the connections they use tend to
be relatively long lived; for disk storage, a host typically opens a
SCSI connection on boot and closes it on shutdown. Tape session
length tends to be measured in hours or fractions thereof (i.e.,
rapid fire sharing of the same tape device among different initiators
is unusual), although tape robot control sessions can be short when
the robot is shared among tape drives. On the other hand, tape will
not see a large number of initiator connections to a single target or
communication endpoint, as each tape drive is dedicated to a single
use at a single time, and a dozen tape drives is a large tape device.
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RFC 3723 Securing Block Storage Protocols over IP April 20042.3. Security Protocol2.3.1. Transforms
All IP block storage security compliant implementations MUST support
IPsec ESP [RFC2406] to provide security for both control packets and
data packets, as well as the replay protection mechanisms of IPsec.
When ESP is utilized, per-packet data origin authentication,
integrity and replay protection MUST be used.
To provide confidentiality with ESP, ESP with 3DES in CBC mode
[RFC2451][3DESANSI] MUST be supported, and AES in Counter mode, as
described in [RFC3686], SHOULD be supported. To provide data origin
authentication and integrity with ESP, HMAC-SHA1 [RFC2404] MUST be
supported, and AES in CBC MAC mode with XCBC extensions [RFC3566]
SHOULD be supported. DES in CBC mode SHOULD NOT be used due to its
inherent weakness. ESP with NULL encryption MUST be supported for
authentication.
2.3.2. IPsec Modes
Conformant IP block storage protocol implementations MUST support ESP
[RFC2406] in tunnel mode and MAY implement IPsec with ESP in
transport mode.
2.3.3. IKE
Conformant IP block storage security implementations MUST support IKE
[RFC2409] for peer authentication, negotiation of security
associations, and key management, using the IPsec DOI [RFC2407].
Manual keying MUST NOT be used since it does not provide the
necessary rekeying support. Conformant IP block storage security
implementations MUST support peer authentication using a pre-shared
key, and MAY support certificate-based peer authentication using
digital signatures. Peer authentication using the public key
encryption methods outlined in IKE's sections 5.2 and 5.3 [RFC2409]
SHOULD NOT be used.
Conformant IP block storage security implementations MUST support IKE
Main Mode and SHOULD support Aggressive Mode. IKE Main Mode with
pre-shared key authentication SHOULD NOT be used when either of the
peers use a dynamically assigned IP address. While Main Mode with
pre-shared key authentication offers good security in many cases,
situations where dynamically assigned addresses are used force use of
a group pre-shared key, which is vulnerable to man-in-the-middle
attack.
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When digital signatures are used for authentication, either IKE Main
Mode or IKE Aggressive Mode MAY be used. In all cases, access to
locally stored secret information (pre-shared key, or private key
for digital signing) must be suitably restricted, since compromise of
the secret information nullifies the security properties of the
IKE/IPsec protocols.
When digital signatures are used to achieve authentication, an IKE
negotiator SHOULD use IKE Certificate Request Payload(s) to specify
the certificate authority (or authorities) that are trusted in
accordance with its local policy. IKE negotiators SHOULD check the
pertinent Certificate Revocation List (CRL) before accepting a PKI
certificate for use in IKE's authentication procedures.
The IPsec DOI [RFC2407] provides for several types of identification
data. Within IKE Phase 1, for use within the IDii and IDir payloads,
conformant IP block storage security implementations MUST support the
ID_IPV4_ADDR, ID_IPV6_ADDR (if the protocol stack supports IPv6) and
ID_FQDN Identity Payloads. iSCSI security implementations SHOULD
support the ID_USER_FQDN Identity Payload; other IP block storage
protocols (iFCP, FCIP) SHOULD NOT use the ID_USER_FQDN Identity
Payload. Identities other than ID_IPV4_ADDR and ID_IPV6_ADDR (such
as ID_FQDN or ID_USER_FQDN) SHOULD be employed in situations where
Aggressive mode is utilized along with pre-shared keys and IP
addresses are dynamically assigned. The IP Subnet, IP Address Range,
ID_DER_ASN1_DN, ID_DER_ASN1_GN formats SHOULD NOT be used for IP
block storage protocol security; The ID_KEY_ID Identity Payload MUST
NOT be used. As described in [RFC2407], within Phase 1 the ID port
and protocol fields MUST be set to zero or to UDP port 500. Also, as
noted in [RFC2407]:
When an IKE exchange is authenticated using certificates (of any
format), any ID's used for input to local policy decisions SHOULD
be contained in the certificate used in the authentication of the
exchange.
The Phase 2 Quick Mode exchanges used by IP block storage protocol
implementations MUST explicitly carry the Identity Payload fields
(IDci and IDcr). Each Phase 2 IDci and IDcr Payload SHOULD carry a
single IP address (ID_IPV4_ADDR, ID_IPV6_ADDR) and SHOULD NOT use the
IP Subnet or IP Address Range formats. Other ID payload formats MUST
NOT be used.
Since IPsec acceleration hardware may only be able to handle a
limited number of active IKE Phase 2 SAs, Phase 2 delete messages may
be sent for idle SAs, as a means of keeping the number of active
Phase 2 SAs to a minimum. The receipt of an IKE Phase 2 delete
message MUST NOT be interpreted as a reason for tearing down an IP
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block storage connection. Rather, it is preferable to leave the
connection up, and if additional traffic is sent on it, to bring up
another IKE Phase 2 SA to protect it. This avoids the potential for
continually bringing connections up and down.
2.3.4. Security Policy Configuration
One of the goals of this specification is to enable a high level of
interoperability without requiring extensive configuration. This
section provides guidelines on setting of IKE parameters so as to
enhance the probability of a successful negotiation. It also
describes how information on security policy configuration can be
provided so as to further enhance the chances of success.
To enhance the prospects for interoperability, some of the actions to
consider include:
[1] Transform restriction.
Since support for 3DES-CBC and HMAC-SHA1 is required of all
implementations, offering these transforms enhances the
probability of a successful negotiation. If AES-CTR [RFC3686]
with XCBC-MAC [RFC3566] is supported, this transform combination
will typically be preferred, with 3DES-CBC/HMAC-SHA1 as a
secondary offer.
[2] Group Restriction.
If 3DES-CBC/HMAC-SHA1 is offered, and DH groups are offered,
then it is recommended that a DH group of at least 1024 bits be
offered along with it. If AES-CTR/XCBC-MAC is the preferred
offer, and DH groups are offered, then it is recommended that a
DH group of at least 2048 bits be offered along with it, as
noted in [KeyLen]. If perfect forward secrecy is required in
Quick Mode, then it is recommended that the QM PFS DH group be
the same as the IKE Phase 1 DH group. This reduces the total
number of combinations, enhancing the chances for
interoperability.
[3] Key lifetimes.
If a key lifetime is offered that is longer than desired, then
rather than causing the IKE negotiation to fail, it is
recommended that the Responder consider the offered lifetime as
a maximum, and accept it. The key can then use a lesser value
for the lifetime, and utilize a Lifetime Notify in order to
inform the other peer of lifetime expiration.
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Even when the above advice is taken, it still may be useful to be
able to provide additional configuration information in order to
enhance the chances of success, and it is useful to be able to manage
security configuration regardless of the scale of the deployment.
For example, it may be desirable to configure the security policy of
an IP block storage device. This can be done manually or
automatically via a security policy distribution mechanism.
Alternatively, it can be supplied via iSNS or SLPv2. If an IP block
storage endpoint can obtain the required security policy by other
means (manually, or automatically via a security policy distribution
mechanism) then it need not request this information via iSNS or
SLPv2. However, if the required security policy configuration is not
available via other mechanisms, iSNS or SLPv2 can be used to obtain
it.
It may also be helpful to obtain information about the preferences of
the peer prior to initiating IKE. While it is generally possible to
negotiate security parameters within IKE, there are situations in
which incompatible parameters can cause the IKE negotiation to fail.
The following information can be provided via SLPv2 or iSNS:
[4] IPsec or cleartext support.
The minimum piece of peer configuration required is whether an
IP block storage endpoint requires IPsec or cleartext. This
cannot be determined from the IKE negotiation alone without
risking a long timeout, which is highly undesirable for a disk
access protocol.
[5] Perfect Forward Secrecy (PFS) support.
It is helpful to know whether a peer allows PFS, since an IKE
Phase 2 Quick Mode can fail if an initiator proposes PFS to a
Responder that does not allow it.
[6] Preference for tunnel mode.
While it is legal to propose both transport and tunnel mode
within the same offer, not all IKE implementations will support
this. As a result, it is useful to know whether a peer prefers
tunnel mode or transport mode, so that it is possible to
negotiate the preferred mode on the first try.
[7] Main Mode and Aggressive Mode support.
Since the IKE negotiation can fail if a mode is proposed to a
peer that doesn't allow it, it is helpful to know which modes a
peer allows, so that an allowed mode can be negotiated on the
first try.
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Since iSNS or SLPv2 can be used to distribute IPsec security policy
and configuration information for use with IP block storage
protocols, these discovery protocols would constitute a 'weak link'
were they not secured at least as well as the protocols whose
security they configure. Since the major vulnerability is packet
modification and replay, when iSNS or SLPv2 are used to distribute
security policy or configuration information, at a minimum, per-
packet data origin authentication, integrity and replay protection
MUST be used to protect the discovery protocol.
2.4. iSCSI Authentication2.4.1. CHAP
Compliant iSCSI implementations MUST implement the CHAP
authentication method [RFC1994] (according to [RFC3720], section11.1.4), which includes support for bi-directional authentication,
and the target authentication option.
When CHAP is performed over non-encrypted channel, it is vulnerable
to an off-line dictionary attack. Implementations MUST support
random CHAP secrets of up to 128 bits, including the means to
generate such secrets and to accept them from an external generation
source. Implementations MUST NOT provide secret generation (or
expansion) means other than random generation.
If CHAP is used with secret smaller than 96 bits, then IPsec
encryption (according to the implementation requirements in [RFC3720]
section 8.3.2) MUST be used to protect the connection. Moreover, in
this case IKE authentication with group pre-shared keys SHOULD NOT be
used. When CHAP is used with a secret smaller then 96 bits, a
compliant implementation MUST NOT continue with the iSCSI login
unless it can verify that IPsec encryption is being used to protect
the connection.
Originators MUST NOT reuse the CHAP challenge sent by the Responder
for the other direction of a bidirectional authentication.
Responders MUST check for this condition and close the iSCSI TCP
connection if it occurs.
The same CHAP secret SHOULD NOT be configured for authentication of
multiple initiators or multiple targets, as this enables any of them
to impersonate any other one of them, and compromising one of them
enables the attacker to impersonate any of them. It is recommended
that iSCSI implementations check for use of identical CHAP secrets by
different peers when this check is feasible, and take appropriate
measures to warn users and/or administrators when this is detected.
A single CHAP secret MAY be used for authentication of an individual
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RFC 3723 Securing Block Storage Protocols over IP April 2004
initiator to multiple targets. Likewise, a single CHAP secret MAY be
used for authentication of an individual target to multiple
initiators.
A Responder MUST NOT send its CHAP response if the initiator has not
successfully authenticated. For example, the following exchange:
I->R CHAP_A=<A1,A2,...>
R->I CHAP_A=<A1> CHAP_C=<C> CHAP_I=<I>
I->R CHAP_N=<N> CHAP_C=<C> CHAP_I=<I>
(Where N, (A1,A2), I, C, and R are correspondingly the Name,
Algorithms, Identifier, Challenge, and Response as defined in
[RFC1994])
MUST result in the Responder (target) closing the iSCSI TCP
connection because the initiator has failed to authenticate (there is
no CHAP_R in the third message).
Any CHAP secret used for initiator authentication MUST NOT be
configured for authentication of any target, and any CHAP secret used
for target authentication MUST NOT be configured for authentication
of any initiator. If the CHAP response received by one end of an
iSCSI connection is the same as the CHAP response that the receiving
endpoint would have generated for the same CHAP challenge, the
response MUST be treated as an authentication failure and cause the
connection to close (this ensures that the same CHAP secret is not
used for authentication in both directions). Also, if an iSCSI
implementation can function as both initiator and target, different
CHAP secrets and identities MUST be configured for these two roles.
The following is an example of the attacks prevented by the above
requirements:
Rogue wants to impersonate Storage to Alice, and knows that a
single secret is used for both directions of Storage-Alice
authentication.
Rogue convinces Alice to open two connections to Rogue, and
Rogue identifies itself as Storage on both connections.
Rogue issues a CHAP challenge on connection 1, waits for Alice
to respond, and then reflects Alice's challenge as the initial
challenge to Alice on connection 2.
If Alice doesn't check for the reflection across connections,
Alice's response on connection 2 enables Rogue to impersonate
Storage on connection 1, even though Rogue does not know the
Alice-Storage CHAP secret.
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Note that RADIUS [RFC2865] does not support bi-directional CHAP
authentication. Therefore, while a target acting as a RADIUS client
will be able to verify the initiator Response, it will not be able to
respond to an initiator challenge unless it has access to an
appropriate shared secret by some other means.
2.4.2. SRP
iSCSI implementations MAY implement the SRP authentication method
[RFC2945] (see [RFC3720], Section 11.1.3). The strength of SRP
security is dependent on the characteristics of the group being used
(i.e., the prime modulus N and generator g). As described in
[RFC2945], N is required to be a Sophie-German prime (of the form N =
2q + 1, where q is also prime) the generator g is a primitive root of
GF(n) [SRPNDSS].
SRP well-known groups are included in Appendix A and additional
groups may be registered with IANA. iSCSI implementations MUST use
one of these well-known groups. All the groups specified in AppendixA up to 1536 bits (i.e., SRP-768, SRP-1024, SRP-1280, SRP-1536) MUST
be supported by initiators and targets. To guarantee
interoperability, targets MUST always offer "SRP-1536" as one of the
proposed groups.
2.5. SLPv2 Security
Both iSCSI and FCIP protocols use SLPv2 as a way to discover peer
entities and management servers. SLPv2 may also be used to provide
information on peer security configuration. When SLPv2 is deployed,
the SA advertisements as well as UA requests and/or responses are
subject to the following security threats:
[1] An attacker could insert or alter SA advertisements or a
response to a UA request in order to masquerade as the real peer
or launch a denial of service attack.
[2] An attacker could gain knowledge about an SA or a UA through
snooping, and launch an attack against the peer. Given the
potential value of iSCSI targets and FCIP entities, leaking of
such information not only increases the possibility of an attack
over the network; there is also the risk of physical theft.
[3] An attacker could spoof a DAAdvert. This could cause UAs and
SAs to use a rogue DAs.
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To address these threats, the following capabilities are required:
[a] Service information, as included in SrvRply, AttrRply, SrvReg
and SrvDereg messages, needs to be kept confidential.
[b] The UA has to be able to distinguish between legitimate and
illegitimate service information from SrvRply and AttrRply
messages. In the SLPv2 security model SAs are trusted to sign
data.
[c] The DA has to be able to distinguish between legitimate and
illegitimate SrvReg and SrvDereg messages.
[d] The UA has to be able to distinguish between legitimate and
illegitimate DA Advertisements. This allows the UA to avoid
rogue DAs that will return incorrect data or no data at all. In
the SLPv2 security model, UAs trust DAs to store, answer queries
on and forward data on services, but not necessarily to
originate it.
[e] SAs may have to trust DAs, especially if 'mesh-enhanced' SLPv2
is used. In this case, SAs register with only one DA and trust
that this DA will forward the registration to others.
By itself, SLPv2 security, defined in [RFC2608], does not satisfy
these security requirements. SLPv2 only provides end-to-end
authentication, but does not support confidentiality. In SLPv2
authentication there is no way to authenticate "zero result
responses". This enables an attacker to mount a denial of service
attack by sending UAs a "zero results" SrvRply or AttrRply as if from
a DA with whose source address corresponds to a legitimate DAAdvert.
In all cases, there is a potential for denial of service attack
against protocol service providers, but such an attack is possible
even in the absence of SLPv2 based discovery mechanisms.
2.5.1. SLPv2 Security Protocol
SLPv2 message types include: SrvRqst, SrvRply, SrvReg, SrvDereg,
SrvAck, AttrRqst, AttrRply, DAAdvert, SrvTypeRqst, SrvTypeRply,
SAAdvert. SLPv2 requires that User Agents (UAs) and Service Agents
(SAs) support SrvRqst, SrvRply, and DAAdvert. SAs must additionally
support SrvReg, SrvAck, and SAAdvert.
Where no Directory Agent (DA) exists, the SrvRqst is multicast, but
the SrvRply is sent via unicast UDP. DAAdverts are also multicast.
However, all other SLPv2 messages are sent via UDP unicast.
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In order to provide the required security functionality, iSCSI and
FCIP implementations supporting SLPv2 security SHOULD protect SLPv2
messages sent via unicast using IPsec ESP with a non-null transform.
SLPv2 authentication blocks (carrying digital signatures), described
in [RFC2608] MAY also be used to authenticate unicast and multicast
messages.
The usage of SLPv2 by iSCSI is described in [iSCSISLP]. iSCSI
initiators and targets may enable IKE mechanisms to establish
identity. In addition, a subsequent user-level iSCSI session login
can protect the initiator-target nexus. This will protect them from
any compromise of security in the SLPv2 discovery process.
The usage of SLPv2 by FCIP is described in [FCIPSLP]. FCIP Entities
assume that once the IKE identity of a peer is established, the FCIP
Entity Name carried in FCIP Short Frame is also implicitly accepted
as the authenticated peer. Any such association between the IKE
identity and the FCIP Entity Name is administratively established.
For use in securing SLPv2, when digital signatures are used to
achieve authentication in IKE, an IKE negotiator SHOULD use IKE
Certificate Request Payload(s) to specify the certificate authority
(or authorities) that are trusted in accordance with its local
policy. IKE negotiators SHOULD check the pertinent Certificate
Revocation List (CRL) before accepting a PKI certificate for use in
IKE's authentication procedures. If key management of SLPv2 DAs
needs to be coordinated with the SAs and the UAs as well as the
protocol service implementations, one may use certificate based key
management, with a shared root Certificate Authority (CA).
One of the reasons for utilizing IPsec for SLPv2 security is that is
more likely that certificates will be deployed for IPsec than for
SLPv2. This both simplifies SLPv2 security and makes it more likely
that it will be implemented interoperably and more importantly, that
it will be used. As a result, it is desirable that little additional
effort be required to enable IPsec protection of SLPv2.
However, just because a certificate is trusted for use with IPsec
does not necessarily imply that the host is authorized to perform
SLPv2 operations. When using IPsec to secure SLPv2, it may be
desirable to distinguish between certificates appropriate for use by
UAs, SAs, and DAs. For example, while a UA might be allowed to use
any certificate conforming to IKE certificate policy, the certificate
used by an SA might indicate that it is a legitimate source of
service advertisements. Similarly, a DA certificate might indicate
that it is a valid DA. This can be accomplished by using special CAs
to issue certificates valid for use by SAs and DAs; alternatively, SA
and DA authorizations can be employed.
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RFC 3723 Securing Block Storage Protocols over IP April 2004
Assume that the policy for issuing and distributing SLPv2 authorized
certificates to SAs and DAs limits them only to legitimate SAs and
DAs. In this case, IPsec is used to provide SLPv2 security as
follows:
[a] SLPv2 messages sent via unicast are IPsec protected, using ESP
with a non-null transform.
[b] SrvRply and AttrRply messages from either a DA or SA are unicast
to UAs. Assuming that the SA used a certificate authorized for
SLPv2 service advertisement in establishing the IKE Phase 1 SA,
or that the DA used a certificate authorized for DA usage, the
UA can accept the information sent, even if it has no SLPv2
authentication block.
Note that where SrvRqst messages are multicast, they are not
protected. An attacker may attempt to exploit this by spoofing
a multicast SrvRqst from the UA, generating a SrvRply from an SA
of the attacker's choosing. Although the SrvRply is secured, it
does not correspond to a legitimate SrvRqst sent by the UA. To
avoid this attack, where SrvRqst messages are multicast, the UA
MUST check that SrvRply messages represent a legitimate reply to
the SrvRqst that was sent.
[c] SrvReg and SrvDereg messages from a SA are unicast to DAs.
Assuming that the SA used a certificate authorized for SLPv2
service advertisement in establishing the IKE Phase 1 SA, the DA
can accept the de/registration even if it has no SLPv2
authentication block. Typically, the SA will check the DA
authorization prior to sending the service advertisement.
[d] Multicast DAAdverts can be considered advisory. The UA will
attempt to contact DAs via unicast. Assuming that the DA used a
certificate authorized for SLPv2 DAAdverts in establishing the
IKE Phase 1 SA, the UA can accept the DAAdvert even if it has no
SLPv2 authentication block.
[e] SAs can accept DAAdverts as described in [d].
2.5.2. Confidentiality of Service Information
Since SLPv2 messages can contain information that can potentially
reveal the vendor of the device or its other associated
characteristics, revealing service information constitutes a security
risk. As an example, the FCIP Entity Name may reveal a WWN from
which an attacker can learn potentially useful information about the
Entity's characteristics.
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The SLPv2 security model assumes that service information is public,
and therefore does not provide for confidentiality. However, storage
devices represent mission critical infrastructure of substantial
value, and so iSCSI and FCIP security implementations supporting
SLPv2 security SHOULD encrypt as well as authenticate and integrity-
protect unicast SLPv2 messages.
Assuming that all unicast SLPv2 messages are protected by IPsec, and
that confidentiality is provided, then the risk of disclosure can be
limited to SLPv2 messages sent via multicast, namely the SrvRqst and
DAAdvert.
The information leaked in a multicast SrvRqst depends on the level of
detail in the query. If leakage is a concern, then a DA can be
provided. If this is not feasible, then a general query can be sent
via multicast, and then further detail can be obtained from the
replying entities via additional unicast queries, protected by IPsec.
Information leakage via a multicast DAAdvert is less of a concern
than the authenticity of the message, since knowing that a DA is
present on the network only enables an attacker to know that SLPv2 is
in use, and possibly that a directory service is also present. This
information is not considered very valuable.
2.5.3. SLPv2 Security Implications
Through the definition of security attributes, it is possible to use
SLPv2 to distribute information about security settings for IP block
storage entities. SLPv2 distribution of security policy is not
necessary if the security settings can be determined by other means,
such as manual configuration or IPsec security policy distribution.
If an entity has already obtained its security configuration via
other mechanisms, then it MUST NOT request security policy via SLPv2.
Where SLPv2 is used to provide security policy information for use
with IP block storage protocols, SLPv2 MUST be protected by IPsec as
described in this document. Where SLPv2 is not used to distribute
security policy information, implementations MAY implement SLPv2
security as described in this document.
Where SLPv2 is used, but security is not implemented, IP block
storage protocol implementations MUST support a negative cache for
authentication failures. This allows implementations to avoid
continually contacting discovered endpoints that fail authentication
within IPsec or at the application layer (in the case of iSCSI
Login). The negative cache need not be maintained within the IPsec
implementation, but rather within the IP block storage protocol
implementation.
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Since this document proposes that hop-by-hop security be used as the
primary mechanism to protect SLPv2, UAs have to trust DAs to
accurately relay data from SAs. This is a change to the SLPv2
security model described in [RFC2608]. However, SLPv2 authentication
as defined in [RFC2608] does not provide a way to authenticate "zero
result responses", leaving SLPv2 vulnerable to a denial of service
attack. Such an attack can be carried out on a UA by sending it a
"zero results" SrvRply or AttrRply, sent from a source address
corresponding to a DA issuing a legitimate DAAdvert.
In addition, SLPv2 security as defined in [RFC2608] does not support
confidentiality. When IPsec with ESP and a non-null transform is
used to protect SLPv2, not only can unicast requests and replies be
authenticated, but confidentiality can also be provided. This
includes unicast requests to DAs and SAs as well as replies. It is
also possible to actively discover SAs using multicast SA discovery,
and then to send unicast requests to the discovered SAs.
As a result, for use with IP block storage protocols, it is believed
that use of IPsec for security is more appropriate than the SLPv2
security model defined in [RFC2608].
Using IPsec to secure SLPv2 has performance implications. Security
associations established between:
- UAs and SAs may be reused (the client on the UA host will use the
service on the SA host).
- SAs and DAs may be reused (the SAs will reregister services)
- UAs and DAs will probably not be reused (many idle security
associations are likely to result, and build up on the DA).
When IPsec is used to protect SLPv2, it is not necessarily
appropriate for all hosts with whom an IPsec security association can
be established to be trusted to originate SLPv2 service
advertisements. This is particularly the case in environments where
it is easy to obtain certificates valid for use with IPsec (for
example, where anyone with access to the network can obtain a machine
certificate valid for use with IPsec). If not all hosts are
authorized to originate service advertisements, then it is necessary
to distinguish between authorized and unauthorized hosts.
This can be accomplished by the following mechanisms:
[1] Configuring SAs with the identities or certificate
characteristics of valid DAs, and configuring DAs with the
identities of SAs allowed to advertise IP block storage
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RFC 3723 Securing Block Storage Protocols over IP April 2004
services. The DAs are then trusted to enforce policies on
service registration. This approach involves manual
configuration, but avoids certificate customization for SLPv2.
[2] Restricting the issuance of certificates valid for use in SLPv2
service advertisement. While all certificates allowed for use
with IPsec will chain to a trusted root, certificates for hosts
authorized to originate service advertisements could be signed
by an SLPv2-authorized CA, or could contain explicit SLPv2
authorizations within the certificate. After the IPsec security
association is set up between the SLPv2 entities, the SLPv2
implementations can then retrieve the certificates used in the
negotiation in order to determine whether the entities are
authorized for the operations that are being performed. This
approach requires less configuration, but requires some
certificate customization for use with SLPv2.
2.6. iSNS Security
The iSCSI protocol may use iSNS for discovery and management
services, while the iFCP protocol is required to use iSNS for such
services. In addition, iSNS can be used to store and distribute
security policy and authorization information to iSCSI and iFCP
devices. When the iSNS protocol is deployed, the interaction between
iSNS server and iSNS clients are subject to the following additional
security threats:
[1] An attacker can alter iSNS protocol messages, directing iSCSI
and iFCP devices to establish connections with rogue devices, or
weakening IPsec protection for iSCSI or iFCP traffic.
[2] An attacker can masquerade as the real iSNS server by sending
false iSNS heartbeat messages. This could deceive iSCSI and
iFCP devices into using rogue iSNS servers.
[3] An attacker can gain knowledge about iSCSI and iFCP devices by
snooping iSNS protocol messages. Such information could aid an
attacker in mounting a direct attack on iSCSI and iFCP devices,
such as a denial-of-service attack or outright physical theft.
To address these threats, the following capabilities are needed:
[a] Unicast iSNS protocol messages may need to be authenticated. In
addition, to protect against threat [3] above, confidentiality
support is desirable, and REQUIRED when certain functions of
iSNS are used.
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[b] Multicast iSNS protocol messages such as the iSNS heartbeat
message need to be authenticated. These messages need not be
confidential since they do not leak critical information.
There is no requirement that the identities of iSNS entities be kept
confidential. Specifically, the identity and location of the iSNS
server need not be kept confidential.
In order to protect against an attacker masquerading as an iSNS
server, client devices MUST support authentication of broadcast or
multicast messages such as the iSNS heartbeat. The iSNS
authentication block (which is identical in format to the SLP
authentication block) MAY be used for this purpose. Note that the
authentication block is used only for iSNS broadcast or multicast
messages, and SHOULD NOT be used in unicast iSNS messages.
Since iSNS is used to distribute authorizations determining which
client devices can communicate, IPsec authentication and data
integrity MUST be supported. In addition, if iSNS is used to
distribute security policy for iFCP and iSCSI devices, then
authentication, data integrity, and confidentiality MUST be supported
and used.
Where iSNS is used without security, IP block storage protocol
implementations MUST support a negative cache for authentication
failures. This allows implementations to avoid continually
contacting discovered endpoints that fail authentication within IPsec
or at the application layer (in the case of iSCSI Login). The
negative cache need not be maintained within the IPsec
implementation, but rather within the IP block storage protocol
implementation.
2.6.1. Use of iSNS to Discover Security Configuration of Peer Devices
In practice, within a single installation, iSCSI and/or iFCP devices
may have different security settings. For example, some devices may
be configured to initiate secure communication, while other devices
may be configured to respond to a request for secure communication,
but not to require security. Still other devices, while security
capable, may neither initiate nor respond securely.
In practice, these variations in configuration can result in devices
being unable to communicate with each other. For example, a device
that is configured to always initiate secure communication will
experience difficulties in communicating with a device that neither
initiates nor responds securely.
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The iSNS protocol is used to transfer naming, discovery, and
management information between iSCSI devices, iFCP gateways,
management stations, and the iSNS server. This includes the ability
to enable discovery of security settings used for communication via
the iSCSI and/or iFCP protocols.
The iSNS server stores security settings for each iSCSI and iFCP
device interface. These security settings, which can be retrieved by
authorized hosts, include use or non-use of IPsec, IKE, Main Mode,
Aggressive Mode, PFS, Pre-shared Key, and certificates.
For example, IKE may not be enabled for a particular device
interface. If a peer device can learn of this in advance by
consulting the iSNS server, it will not need to waste time and
resources attempting to initiate an IKE Phase 1 SA with that device
interface.
If iSNS is used to distribute security policy, then the minimum
information that should be learned from the iSNS server is the use or
non-use of IKE and IPsec by each iFCP or iSCSI peer device interface.
This information is encoded in the Security Bitmap field of each
Portal of the peer device, and is applicable on a per-interface basis
for the peer device. iSNS queries to acquire security configuration
data about peer devices MUST be protected by IPsec/ESP
authentication.
2.6.2. Use of iSNS to Distribute iSCSI and iFCP Security Policies
Once communication between iSNS clients and the iSNS server are
secured through use of IPsec, iSNS clients have the capability to
discover the security settings required for communication via the
iSCSI and/or iFCP protocols. Use of iSNS for distribution of
security policies offers the potential to reduce the burden of manual
device configuration, and decrease the probability of communications
failures due to incompatible security policies. If iSNS is used to
distribute security policies, then IPsec authentication, data
integrity, and confidentiality MUST be used to protect all iSNS
protocol messages.
The complete IKE/IPsec configuration of each iFCP and/or iSCSI device
can be stored in the iSNS server, including policies that are used
for IKE Phase 1 and Phase 2 negotiations between client devices. The
IKE payload format includes a series of one or more proposals that
the iSCSI or iFCP device will use when negotiating the appropriate
IPsec policy to use to protect iSCSI or iFCP traffic.
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Note that iSNS distribution of security policy is not necessary if
the security settings can be determined by other means, such as
manual configuration or IPsec security policy distribution. If an
entity has already obtained its security configuration via other
mechanisms, then it MUST NOT request security policy via iSNS.
For further details on how to store and retrieve IKE policy proposals
in the iSNS server, see [iSNS].
2.6.3. iSNS Interaction with IKE and IPsec
When IPsec security is enabled, each iSNS client that is registered
in the iSNS database maintains at least one Phase 1 and one Phase 2
security association with the iSNS server. All iSNS protocol
messages between iSNS clients and the iSNS server are to be protected
by a phase-2 security association.
2.6.4. iSNS Server Implementation Requirements
All iSNS implementations MUST support the replay protection
mechanisms of IPsec. ESP in tunnel mode MUST be implemented, and
IPsec with ESP in transport mode MAY be implemented.
To provide data origin authentication and integrity with ESP, HMAC-
SHA1 MUST be supported, and AES in CBC MAC mode with XCBC extensions
[RFC3566] SHOULD be supported. When confidentiality is implemented,
3DES in CBC mode MUST be supported, and AES in Counter mode, as
described in [RFC3686], SHOULD be supported. DES in CBC mode SHOULD
NOT be used due to its inherent weakness. If confidentiality is not
required but data origin authentication and integrity is enabled, ESP
with NULL Encryption MUST be used.
Conformant iSNS implementations MUST support IKE for authentication,
negotiation of security associations, and key management, using the
IPsec DOI, described in [RFC2407]. IP block storage protocols can be
expected to send data in high volumes, thereby requiring rekey.
Since manual keying does not provide rekeying support, its use is
prohibited with IP block storage protocols. Although iSNS does not
send a high volume of data, and therefore rekey is not a major
concern, manual keying SHOULD NOT be used. This is for consistency,
since dynamic keying support is already required in IP storage
security implementations.
Conformant iSNS security implementations MUST support authentication
using a pre- shared key, and MAY support certificate-based peer
authentication using digital signatures. Peer authentication using
the public key encryption methods outlined in [RFC2409] sections 5.2
and 5.3 SHOULD NOT be used.
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Conformant iSNS implementations MUST support IKE Main Mode and SHOULD
support Aggressive Mode. IKE Main Mode with pre-shared key
authentication SHOULD NOT be used when either of the peers use
dynamically assigned IP addresses. While Main Mode with pre-shared
key authentication offers good security in many cases, situations
where dynamically assigned addresses are used force use of a group
pre-shared key, which is vulnerable to man-in-the-middle attack.
When digital signatures are used for authentication, either IKE Main
Mode or IKE Aggressive Mode MAY be used. In all cases, access to
locally stored secret information (pre-shared key or private key for
digital signing) MUST be suitably restricted, since compromise of the
secret information nullifies the security properties of the IKE/IPsec
protocols.
When digital signatures are used to achieve authentication, an IKE
negotiator SHOULD use IKE Certificate Request Payload(s) to specify
the certificate authority (or authorities) that are trusted in
accordance with its local policy. IKE negotiators SHOULD check the
pertinent Certificate Revocation List (CRL) before accepting a PKI
certificate for use in IKE's authentication procedures.
3. iSCSI Security Interoperability Guidelines
The following guidelines are established to meet iSCSI security
requirements using IPsec in practical situations.
3.1. iSCSI Security Issues
iSCSI provides for iSCSI Login, outlined in [RFC3720], which includes
support for application-layer authentication. This authentication is
logically between the iSCSI initiator and the iSCSI target (as
opposed to between the TCP/IP communication endpoints). The intent
of the iSCSI design is that the initiator and target represent the
systems (e.g., host and disk array or tape system) participating in
the communication, as opposed to network communication interfaces or
endpoints.
The iSCSI protocol and iSCSI Login authentication do not meet the
security requirements for iSCSI. iSCSI Login authentication provides
mutual authentication between the iSCSI initiator and target at
connection origination, but does not protect control and data traffic
on a per packet basis, leaving the iSCSI connection vulnerable to
attack. iSCSI Login authentication can mutually authenticate the
initiator to the target, but does not by itself provide per-packet
authentication, integrity, confidentiality or replay protection. In
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RFC 3723 Securing Block Storage Protocols over IP April 2004
addition, iSCSI Login authentication does not provide for a protected
ciphersuite negotiation. Therefore, iSCSI Login provides a weak
security solution.
3.2. iSCSI and IPsec Interaction
An iSCSI session [RFC3720], comprised of one or more TCP connections,
is identified by the 2-tuple of the initiator-defined identifier and
the target-defined identifier, <ISID, TSIH>. Each connection within
a given session is assigned a unique Connection Identification, CID.
The TCP connection is identified by the 5-tuple <Source IP address,
Destination IP address, Protocol (TCP), Source Port, Destination
Port>. An IPsec Phase 2 SA is identified by the 3-tuple <Protocol
(ESP),destination address, SPI>.
The iSCSI session and connection information is carried within the
iSCSI Login Commands, transported over TCP. Since an iSCSI initiator
may have multiple interfaces, iSCSI connections within an iSCSI
session may be initiated from different IP addresses. Similarly,
multiple iSCSI targets may exist behind a single IP address, so that
there may be multiple iSCSI sessions between a given <source IP
address, destination IP address> pair.
When multiple iSCSI sessions are active between a given <initiator,
target> pair, the set of TCP connections used by a given iSCSI
session must be disjoint from those used by all other iSCSI sessions
between the same <initiator, target> pair. Therefore a TCP
connection can be associated with one and only one iSCSI session.
The relationship between iSCSI sessions, TCP connections and IKE
Phase 1 and Phase 2 SAs is as follows:
[1] An iSCSI initiator or target may have more than one interface,
and therefore may have multiple IP addresses. Also, multiple
iSCSI initiators and targets may exist behind a single IP
address. As a result, an iSCSI Session may correspond to
multiple IKE Phase 1 Security Associations, though typically a
single IKE Phase 1 security association will exist for an
<initiator IP address, target IP address> tuple.
[2] Each TCP connection within an iSCSI Session is protected by an
IKE Phase 2 SA. The selectors may be specific to that TCP
connection or may cover multiple connections. While each IKE
Phase 2 SA may protect multiple TCP connections, each TCP
connection is transported under only one IKE Phase 2 SA.
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Given this, all the information needed for the iSCSI/IPsec binding is
contained within the iSCSI Login messages from the iSCSI initiator
and target. This includes the binding between an IKE Phase 1 SA and
the corresponding iSCSI sessions, as well as the binding between a
TCP connection, an IKE Phase 2 SA and the iSCSI connection ID.
3.3. Initiating a New iSCSI Session
In order to create a new iSCSI Session, if an IKE Phase 1 SA does not
already exist, then it is established by an initiator implementing
iSCSI security. Subsequent iSCSI connections established within the
iSCSI session will typically be protected by IKE Phase 2 SAs derived
from that IKE Phase 1 SA, although additional IKE Phase 1 SAs can
also be brought up.
The initiator and target implementations successfully complete the
IKE Phase 1 and Phase 2 negotiations before the iSCSI initiator
contacts the target on well-known TCP port 3260, and sends the iSCSI
Login command over the TCP connection. IPsec implementations
configured with the correct policies (e.g., use ESP with non-null
transform for all traffic destined for the iSCSI well-known TCP port
3260) will handle this automatically.
The initiator fills in the ISID field, and leaves the TSIH field set
to zero, to indicate that it is the first message of a new session
establishment exchange. The initiator also fills in a CID value,
that identifies the TCP connection over which the Login command is
being exchanged. When the iSCSI target replies with its Login
Command, both iSCSI devices will know the TSIH, and therefore the
iSCSI session identifier <ISID, TSIH>.
A single iSCSI session identifier may have multiple associated IKE
Phase 1 SAs, and each IKE Phase 1 SA may correspond to multiple iSCSI
session identifiers. Each iSCSI connection (identified by the
connection identifier) corresponds to a single TCP connection
(identified by the 5-tuple). Each IKE Phase 2 SA is identified by
the <Protocol (ESP), destination address, SPI> combination. A Phase
2 SA may protect multiple TCP connections, and corresponds to a
single IKE Phase 1 SA.
Within IKE, each key refresh requires that a new security association
be established. In practice there is a time interval during which an
old, about-to-expire SA and newly established SA will both be valid.
The IPsec implementation will choose which security association to
use based on local policy, and iSCSI concerns play no role in this
selection process.
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RFC 3723 Securing Block Storage Protocols over IP April 20043.4. Graceful iSCSI Teardown
Mechanisms within iSCSI provide for both graceful and non-graceful
teardown of iSCSI Sessions or individual TCP connections within a
given session. The iSCSI Logout command is used to effect graceful
teardown. This command allows the iSCSI initiator to request that:
[a] the session be closed
[b] a specific connection within the session be closed
[c] a specific connection be marked for recovery
When the iSCSI implementation wishes to close a session, it uses the
appropriate iSCSI commands to accomplish this. After exchanging the
appropriate iSCSI control messages for session closure, the iSCSI
security implementation will typically initiate a half-close of each
TCP connection within the iSCSI session.
When the iSCSI security implementation wishes to close an individual
TCP connection while leaving the parent iSCSI session active, it
should half-close the TCP connection. After the expiration of the
TIME_WAIT timeout, the TCP connection is closed.
3.5. Non-graceful iSCSI Teardown
If a given TCP connection unexpectedly fails, the associated iSCSI
connection is torn down. There is no requirement that an IKE Phase 2
delete immediately follow iSCSI connection tear down or Phase 1
deletion. Since an IKE Phase 2 SA may correspond to multiple TCP
connections, such a deletion might be inappropriate. Similarly, if
the IKE implementation receives a Phase 2 Delete message for a
security association corresponding to a TCP connection, this does not
necessarily imply that the TCP or iSCSI connection is to be torn
down.
If a Logout Command/Logout Response sequence marks a connection for
removal from the iSCSI session, then after the iSCSI peer has
executed an iSCSI teardown process for the connection, the TCP
connection will be closed. The iSCSI connection state can then be
safely removed.
Since an IKE Phase 2 SA may be used by multiple TCP connections, an
iSCSI implementation should not depend on receiving the IPsec Phase 2
delete as confirmation that the iSCSI peer has executed an iSCSI
teardown process for the connection.
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Since IPsec acceleration hardware may only be able to handle a
limited number of active IKE Phase 2 SAs, Phase 2 delete messages may
be sent for idle SAs, as a means of keeping the number of active
Phase 2 SAs to a minimum. The receipt of an IKE Phase 2 delete
message MUST NOT be interpreted as a reason for tearing down the
corresponding iSCSI connection if no Logout Command/Logout Receive
has been executed on the connection. Rather, it is preferable to
leave the iSCSI connection up, and if additional traffic is sent on
it, to bring up another IKE Phase 2 SA to protect it. This avoids
the potential for continually bringing iSCSI connections up and down.
3.6. Application-Layer CRC
iSCSI's error detection and recovery assumes that the TCP and IP
checksums provide inadequate integrity protection for high speed
communications. As described in [CRCTCP], when operating at high
speeds, the 16-bit TCP checksum [RFC793] will not necessarily detect
all errors, resulting in possible data corruption. iSCSI [RFC3720]
therefore incorporates a 32-bit CRC to protect its headers and data.
When a CRC check fails (i.e., CRC computed at receiver does not match
the received CRC), the iSCSI PDU covered by that CRC is discarded.
Since presumably the error was not detected by the TCP checksum, TCP
retransmission will not occur and thus cannot assist in recovering
from the error. iSCSI contains both data and command retry
mechanisms to deal with the resulting situations, including SNACK,
the ability to reissue R2T commands, and the retry (X) bit for
commands.
IPsec offers protection against an attacker attempting to modify
packets in transit, as well as unintentional packet modifications
caused by network malfunctions or other errors. In general, IPsec
authentication transforms afford stronger integrity protection than
both the 16-bit TCP checksum and the 32-bit application-layer CRC
specified for use with iSCSI. Since IPsec integrity protection
occurs below TCP, if an error is discovered, then the packet will be
discarded and TCP retransmission will occur, so that no recovery
action need be taken at the iSCSI layer.
3.6.1. Simplification of Recovery Logic
Where IPsec integrity protection is known to be in place end-to-end
between iSCSI endpoints (or the portion that requires additional
integrity protection), portions of iSCSI can be simplified. For
example, mechanisms to recover from CRC check failures are not
necessary.
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If the iSCSI CRC is negotiated, the recovery logic can be simplified
to regard any CRC check failure as fatal (e.g., generate a SCSI CHECK
CONDITION on data error, close the corresponding TCP connection on
header error) because it will be very rare for errors undetected by
IPsec integrity protection to be detected by the iSCSI CRC.
3.6.2. Omission of iSCSI CRC
In some situations where IPsec is employed, the iSCSI CRC will not
provide additional protection and can be omitted.
For example, where IPsec processing as well as TCP checksum and iSCSI
CRC verification are offloaded within the NIC, each of these checks
will be verified prior to transferring data across the bus, so that
subsequent errors will not be detected by these mechanisms. As a
result, where IPsec processing is offloaded to the NIC, the iSCSI CRC
is not necessary and the implementations may wish not to negotiate
it.
However, in other circumstances, the TCP checksum and iSCSI CRC will
provide additional error coverage because they are computed and
checked at a different point in the protocol stack or in devices
different from those that implement the IPsec integrity checks. The
resulting coverage of additional possible errors may make it
desirable to negotiate use of the iSCSI CRC even when IPsec integrity
protection is in use. Examples of these situations include where:
[1] IPsec, TCP and iSCSI are implemented purely in software. Here,
additional failure modes may be detected by the TCP checksum
and/or iSCSI CRC. For example, after the IPsec message
integrity check is successfully verified, the segment is copied
as part of TCP processing, and a memory error during this
process might cause the TCP checksum or iSCSI CRC verification
to fail.
[2] The implementation is an iSCSI-iSCSI proxy or gateway. Here the
iSCSI CRC can be propagated from one iSCSI connection to
another. In this case, the iSCSI CRC is useful to protect iSCSI
data against memory, bus, or software errors within the proxy or
gateway, and requesting it is desirable.
[3] IPsec is provided by a device external to the actual iSCSI
device. Here the iSCSI header and data CRCs can be kept across
the part of the connection that is not protected by IPsec. For
instance, the iSCSI connection could traverse an extra bus,
interface card, network, interface card, and bus between the
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iSCSI device and the device providing IPsec. In this case, the
iSCSI CRC is desirable, and the iSCSI implementation behind the
IPsec device may request it.
Note that if both ends of the connection are on the same
segment, then traffic will be effectively protected by the layer
2 CRC, so that negotiation of the iSCSI CRC is not necessary to
protect against NIC and network errors, although it may be
desirable for other reasons (e.g., [1] and [2] above).
4. iFCP and FCIP Security Issues4.1. iFCP and FCIP Authentication Requirements
iFCP and FCIP are peer-to-peer protocols. iFCP and FCIP sessions may
be initiated by either or both peer gateways. Consequently, bi-
directional authentication of peer gateways MUST be provided.
iFCP and FCIP are transport protocols that encapsulate SCSI and Fibre
Channel frames over IP. Therefore, Fibre Channel, operating system,
and user identities are transparent to the iFCP and FCIP protocols.
iFCP gateways use Discovery Domain information obtained from the iSNS
server to determine whether the initiating Fibre Channel N_PORT
should be allowed access to the target N_PORT. N_PORT identities
used in the Port Login (PLOGI) process will be considered
authenticated provided that they are received over a connection whose
security complies with the local security policy.
There is no requirement that the identities used in authentication be
kept confidential.
4.2. iFCP Interaction with IPsec and IKE
A conformant iFCP Portal is capable of establishing one or more IKE
Phase-1 Security Associations (SAs) to a peer iFCP Portal. A Phase-1
SA may be established when an iFCP Portal is initialized, or may be
deferred until the first TCP connection with security requirements is
established.
An IKE Phase-2 SA protects one or more TCP connections within the
same iFCP Portal. More specifically, the successful establishment of
an IKE Phase-2 SA results in the creation of two uni-directional
IPsec SAs fully qualified by the tuple <SPI, destination IP address,
ESP>. These SAs protect the setup process of the underlying TCP
connections and all their subsequent TCP traffic. Each of the TCP
connections protected by an SA is either in the unbound state, or is
bound to a specific iFCP session.
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In summary, at any point in time:
[1] There exist 0..M IKE Phase-1 SAs between peer iFCP portals
[2] Each IKE Phase-1 SAs has 0..N IKE Phase-2 SAs
[3] Each IKE Phase-2 SA protects 0..Z TCP connections
The creation of an IKE Phase-2 SA may be triggered by security policy
rules retrieved from an iSNS server. Alternately, the creation of an
SA may be triggered by policy rules configured through a management
interface, reflecting iSNS-resident policy rules. Likewise, the use
of a Key Exchange payload in Quick Mode for perfect forward secrecy
may be driven by security policy rules retrieved from the iSNS
server, or set through a management interface.
If an iFCP implementation makes use of unbound TCP connections, and
such connections belong to an iFCP Portal with security requirements,
then the unbound connections MUST be protected by an SA at all times
just like bound connections.
Upon receiving an IKE Phase-2 delete message, there is no requirement
to terminate the protected TCP connections or delete the associated
IKE Phase-1 SA. Since an IKE Phase-2 SA may be associated with
multiple TCP connections, terminating such connections might in fact
be inappropriate and untimely.
To minimize the number of active Phase-2 SAs, IKE Phase-2 delete
messages may be sent for Phase-2 SAs whose TCP connections have not
handled data traffic for a while. To minimize the use of SA
resources while the associated TCP connections are idle, creation of
a new SA should be deferred until new data are to be sent over the
connections.
4.3. FCIP Interaction with IPsec and IKE
FCIP Entities establish tunnels with other FCIP Entities in order to
transfer IP encapsulated FC frames. Each tunnel is a separate FCIP
Link, and can encapsulate multiple TCP connections. The binding of
TCP connections to an FCIP Link is performed using the Fibre Channel
World Wide Names (WWNs) of the two FCIP Entities.
FCIP Entities may have more than one interface and IP address, and it
is possible for an FCIP Link to contain multiple TCP connections
whose FCIP endpoint IP Addresses are different. In this case, an IKE
Phase 1 SA is typically established for each FCIP endpoint IP Address
pair. For the purposes of establishing an IKE Phase 1 SA, static IP
addresses are typically used for identification.
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Each TCP connection within an FCIP Link corresponds to an IKE Phase 2
(Quick Mode) SA. This is established prior to sending the initial
TCP SYN packet and applies to the life of the connection. Phase 2
negotiation is also required for rekeying, in order to protect
against replay attacks.
FCIP implementations MAY provide administrative management of
Confidentiality usage. These management interfaces SHOULD be
provided in a secure manner, so as to prevent an attacker from
subverting the security process by attacking the management
interface.
FCIP Entities do not require any user-level authentication, since all
FCIP Entities participate in a machine-level tunnel function. FCIP
uses SLP for discovery, but not to distribute security policies.
5. Security Considerations5.1. Transport Mode Versus Tunnel Mode
With respect to block storage protocols, the major differences
between the IPsec tunnel mode and transport mode are as follows:
[1] MTU considerations.
Tunnel mode introduces an additional IP header into the datagram
that reflects itself in a corresponding decrease in the path MTU
seen by packets traversing the tunnel. This may result in a
decrease in the maximum segment size of TCP connections running
over the tunnel.
[2] Address assignment and configuration.
Within IPsec tunnel mode, it is necessary for inner and outer
source addresses to be configured, and for inner and outer
destination addresses to be discovered. Within transport mode
it is only necessary to discover a single destination address
and configure a single source address. IPsec tunnel mode
address usage considerations are discussed in more detail below.
[3] NAT traversal.
As noted in [RFC3715], IPsec tunnel mode ESP can traverse NAT in
limited circumstances, whereas transport mode ESP cannot
traverse NAT. To enable NAT traversal in the general case, the
IPsec NAT traversal functionality described in [RFC3715],
[UDPIPsec] and [NATIKE] can be implemented. More details are
provided in the next section.
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RFC 3723 Securing Block Storage Protocols over IP April 2004
[4] Firewall traversal.
Where a block storage protocol is to traverse administrative
domains, the firewall administrator may desire to verify the
integrity and authenticity of each transiting packet, rather
than opening a hole in the firewall for the block storage
protocol. IPsec tunnel mode lends itself to such verification,
since the firewall can terminate the tunnel mode connection
while still allowing the endpoints to communicate end-to-end.
If desired, the endpoints can in addition utilize IPsec
transport mode for end-to-end security, so that they can also
verify authenticity and integrity of each packet for themselves.
In contrast, carrying out this verification with IPsec transport
mode is more complex, since the firewall will need to terminate
the IPsec transport mode connection and will need to act as an
iSCSI, iFCP or FCIP gateway or TCP proxy, originating a new
IPsec transport mode connection from the firewall to the
internal endpoint. Such an implementation cannot provide end-
to-end authenticity or integrity protection, and an
application-layer CRC (iSCSI) or forwarding of the Fibre Channel
frame CRC (iFCP and FCIP) is necessary to protect against errors
introduced by the firewall.
[5] IPsec-application integration.
Where IPsec and the application layer protocol are implemented
on the same device or host, it is possible to enable tight
integration between them. For example, the application layer
can request and verify that connections are protected by IPsec,
and can obtain attributes of the IPsec security association.
While in transport mode implementations the IPsec and
application protocol implementations typically reside on the
same host, with IPsec tunnel mode they may reside on different
hosts. Where IPsec is implemented on an external gateway,
integration between the application and IPsec is typically not
possible. This limits the ability of the application to control
and verify IPsec behavior.
5.1.1. IPsec Tunnel Mode Addressing Considerations
IPsec tunnels include both inner and outer source as well as
destination addresses.
When used with IP block storage protocols, the inner destination
address represents the address of the IP block storage protocol peer
(e.g., the ultimate destination for the packet). The inner
destination address can be discovered using SLPv2 or iSNS, or can be
resolved from an FQDN via DNS, so that obtaining this address is
typically not an issue.
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The outer destination address represents the address of the IPsec
security gateway used to reach the peer. Several mechanisms have
been proposed for discovering the IPsec security gateway used to
reach a particular peer. [RFC2230] discusses the use of KX Resource
Records (RRs) for IPsec gateway discovery. However, while KX RRs are
supported by many DNS server implementations, they have not yet been
widely deployed. Alternatively, DNS SRV [RFC2782] can be used for
this purpose. Where DNS is used for gateway location, DNS security
mechanisms such as DNSSEC ([RFC2535], [RFC2931]), TSIG [RFC2845], and
Simple Secure Dynamic Update [RFC3007] are advisable.
When used with IP block storage protocols, the outer source address
is configured either statically or dynamically, using mechanisms such
as DHCPv4 [RFC2131], DHCPV6 [RFC3315], or stateless address
autoconfiguration [RFC2373].
The inner source address SHOULD be included in the Quick Mode ID
payload when the peer establishes a tunnel mode SA with the IPsec
security gateway. This enables the IPsec security gateway to
properly route packets back to the remote peer. The inner source
address can be configured via a variety of mechanisms, depending on
the scenario. Where the IP block storage peers are located within
the same administrative domain, it is typically possible for the
inner and outer source addresses to be the same. This will work
because the outer source address is presumably assigned from within a
prefix assigned to the administrative domain, and is therefore
routable within the domain. Assuming that the IPsec security gateway
is aware of the inner source address used by the connecting peer and
plumbs a host route for it, then packets arriving at the IPsec
security gateway destined for the address can be correctly
encapsulated and sent down the correct tunnel.
Where IP block storage peers are located within different
administrative domains, it may be necessary for the inner source
address to be assigned by the IPsec security gateway, effectively
"joining" the remote host to the LAN attached to the IPsec security
gateway. For example, if the remote peer were to use its assigned
(outer) source address as the inner source address, then a number of
problems might result:
[1] Intrusion detection systems sniffing the LAN behind the IPsec
security gateway would notice source addresses originating
outside the administrative domain.
[2] Reply packets might not reach their destination, since the IPsec
security gateway typically does not advertise the default route,
but rather only the prefix that it allocates addresses from.
Since the remote peer's address does not originate from with a
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RFC 3723 Securing Block Storage Protocols over IP April 2004
prefix native to the administrative domain, it is likely that
routers within the domain would not have a route for it, and
would send the packet off to the router advertising the default
route (perhaps a border router) instead of to the IPsec security
gateway.
For these reasons, for inter-domain use, assignment of inner source
addresses may be needed. At present this is not a very common
scenario; however, if address assignment is provided, then DHCP-based
address assignment within IPsec tunnel mode [RFC3456] MUST be
supported. Note that this mechanism is not yet widely deployed
within IPsec security gateways; existing IPsec tunnel mode servers
typically implement this functionality via proprietary extensions to
IKE.
5.2. NAT Traversal
As noted in [RFC3715], tunnel mode ESP can traverse NAT in a limited
set of circumstances. For example, if there is only one protocol
endpoint behind a NAT, "ANY to ANY" selectors are negotiated, and the
receiver does not perform source address validation, then tunnel mode
ESP may successfully traverse NATs. Since ignoring source address
validation introduces new security vulnerabilities, and negotiation
of specific selectors is desirable so as to limit the traffic that
can be sent down the tunnel, these conditions may not necessarily
apply, and tunnel mode NAT traversal will not always be possible.
TCP carried within Transport mode ESP cannot traverse NAT, even
though ESP itself does not include IP header fields within its
message integrity check. This is because the 16-bit TCP checksum is
calculated based on a "pseudo-header" that includes IP header fields,
and the checksum is protected by the IPsec ESP message integrity
check. As a result, the TCP checksum will be invalidated by a NAT
located between the two endpoints.
Since TCP checksum calculation and verification is mandatory in both
IPv4 and IPv6, it is not possible to omit checksum verification while
remaining standards compliant. In order to enable traversal of NATs
existing while remaining in compliance, iSCSI, iFCP or FCIP security
implementations can implement IPsec/IKE NAT traversal, as described
in [RFC3715], [UDPIPsec], and [NATIKE].
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The IKE [NATIKE] and IPsec [UDPIPsec] NAT traversal specifications
enable UDP encapsulation of IPsec to be negotiated if a NAT is
detected in the path. By determining the IP address of the NAT, the
TCP checksum can be calculated based on a pseudo-header including the
NAT-adjusted address and ports. If this is done prior to calculating
the IPsec message integrity check, TCP checksum verification will not
fail.
5.3. IKE Issues
There are situations where it is necessary for IKE to be implemented
in firmware. In such situations, it is important to keep the size of
the IKE implementation within strict limits. An upper bound on the
size of an IKE implementation might be considered to be 800 KB, with
80 KB enabling implementation in a wide range of situations.
As noted in Table 5.3-1 on the next page, IKE implementations
currently exist which meet the requirements. Therefore, while
removal of seldom used IKE functionality (such as the nonce
authentication methods) would reduce complexity, implementations
typically will not require this in order to fit within the code size
budget.
5.4. Rekeying Issues
It is expected that IP block storage implementations will need to
operate at high speed. For example, implementations operating at 1
Gbps are currently in design, with 10 Gbps implementations to follow
shortly thereafter. At these speeds, a single IPsec SA could rapidly
cycle through the 32-bit IPsec sequence number space.
For example, a single SA operating at 1 Gbps line rate and sending 64
octet packets would exhaust the 32-bit sequence number space in 2200
seconds, or 37 minutes. With 1518 octet packets, exhaustion would
occur in 14.5 hours. At 10 Gbps, exhaustion would occur in 220
seconds or 3.67 minutes. With 1518 octet packets, this would occur
within 1.45 hours.
In the future, it may be desirable for implementations operating at
speeds of 1 Gbps or greater to implement IPsec sequence number
extension, described in [Seq]. Note that depending on the transform
in use, it is possible that rekeying will be required prior to
exhaustion of the sequence number space.
In CBC-mode ciphers the ciphertext of one block depends on the
plaintext of that block as well as the ciphertext of the previous
block. This implies that if the ciphertext of two blocks are
identical, and the plaintext of the next block is also identical,
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RFC 3723 Securing Block Storage Protocols over IP April 2004
then the ciphertext of the next block will be identical. Thus, if
identical ciphertext blocks can be found with matching subsequent
blocks, an attacker can determine the existence of matching
plaintext.
Such "Birthday attacks" were examined by Bellare et. al. in
[DESANALY]. On average, a ciphertext block of size n bits will be
expected to repeat every 2^[n/2] blocks. Although a single "birthday
attack" does not provide much information to an attacker, multiple
such attacks might provide useful information. To make this
unlikely, it is recommended that a rekey occur before 2^[n/2] blocks
have been sent on a given SA. Bellare et. al. have formalized this
in [DESANALY], showing that the insecurity of CBC mode increases as
O(s^2/2^n), where n is the block size in bits, and s is the total
number of blocks encrypted. These conclusions do not apply to
counter mode.
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This means that cipher block size as well as key length needs to be
considered in the rekey decision. 3DES uses a block size n = 64 bits
(2^3 bytes); this implies that the SA must be rekeyed before B =
(64/8) * (2^32) = 2^35 bytes are sent. At 1 Gbps, this implies that
a rekey will be required every 274.9 seconds (4.6 minutes); at 10
Gbps, a rekey is required every 27.5 seconds.
In terms of the sequence number space, for a 3DES encrypted message
of 512 = 2^9 bytes (2^6 blocks) this implies that the key has become
insecure after about 2^26 messages.
5.5. Transform Issues
Since IP block storage implementations may operate at speeds of 1
Gbps or greater, the ability to offer IPsec security services at high
speeds is of intense concern. Since support for multiple algorithms
multiplies the complexity and expense of hardware design, one of the
goals of the transform selection effort is to find a minimal set of
confidentiality and authentication algorithms implementable in
hardware at speeds of up to 10 Gbps, as well as being efficient for
implementation in software at speeds of 100 Mbps or slower.
In this specification, we primarily concern ourselves with IPsec
transforms that have already been specified, and for which parts are
available that can run at 1 Gbps line rate. Where existing
algorithms do not gracefully scale to 10 Gbps, we further consider
algorithms for which transform specifications are not yet complete,
but for which parts are expected to be available for inclusion in
products shipping within the next 12 months. As the state of the art
advances, the range of feasible algorithms will broaden and
additional mandatory-to-implement algorithms may be considered.
Section 5 of [RFC2406] states:
"A compliant ESP implementation MUST support the following
mandatory-to-implement algorithms:
- DES in CBC mode
- HMAC with MD5
- HMAC with SHA-1
- NULL Authentication algorithm
- NULL Encryption algorithm"
The DES algorithm is specified in [FIPS46-3]; implementation
guidelines are found in [FIPS74], and security issues are discussed
in [DESDIFF],[DESINT], [DESCRACK]. The DES IPsec transform is
defined in [RFC2405] and the 3DES in CBC mode IPsec transform is
specified in [RFC2451].
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The MD5 algorithm is specified in [RFC1321]; HMAC is defined in
[RFC2104] and security issues with MD5 are discussed in [MD5Attack].
The HMAC-MD5 IPsec transform is specified in [RFC2403]. The HMAC-
SHA1 IPsec transform is specified in [RFC2404].
In addition to these existing algorithms, NIST is currently
evaluating the following modes [NSPUE2] of AES, defined in [FIPS197]:
AES in Electronic Codebook (ECB) confidentiality mode
AES in Cipher Block Chaining (CBC) confidentiality mode
AES in Cipher Feedback (CFB) confidentiality mode
AES in Output Feedback (OFB) confidentiality mode
AES in Counter (CTR) confidentiality mode
AES CBC-MAC authentication mode
When utilizing AES modes, it may be necessary to use larger public
keys; the tradeoffs are described in [KeyLen]. Additional MODP
Diffie-Hellman groups for use with IKE are described in [RFC3526].
The Modes [NSPUE2] effort is also considering a number of additional
algorithms, including the following:
PMAC
To provide authentication, integrity and replay protection, IP block
storage security implementations MUST support HMAC-SHA1 [RFC2404] for
authentication, and AES in CBC MAC mode with XCBC extensions SHOULD
be supported [RFC3566].
HMAC-SHA1 [RFC2404] is to be preferred to HMAC-MD5, due to concerns
that have been raised about the security of MD5 [MD5Attack]. HMAC-
SHA1 parts are currently available that run at 1 Gbps, the algorithm
is implementable in hardware at speeds up to 10 Gbps, and an IPsec
transform specification [RFC2404] exists. As a result, it is most
practical to utilize HMAC-SHA1 as the authentication algorithm for
products shipping in the near future. AES in CBC-MAC authentication
mode with XCBC extensions was selected since it avoids adding
substantial additional code if AES is already being implemented for
encryption; an IPsec transform document is available [RFC3566].
Authentication transforms also exist that are considerably more
efficient to implement than HMAC-SHA1, or AES in CBC-MAC
authentication mode. UMAC [UMAC],[UMACKR] is more efficient to
implement in software and PMAC [PMAC] is more efficient to implement
in hardware. PMAC is currently being evaluated as part of the NIST
modes effort [NSPUE2] but an IPsec transform does not yet exist;
neither does a UMAC transform.
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For confidentiality, the ESP mandatory-to-implement algorithm (DES)
is unacceptable. As noted in [DESCRACK], DES is crackable with
modest computation resources, and so is inappropriate for use in
situations requiring high levels of security.
To provide confidentiality for iSCSI, iFCP, and FCIP, 3DES in CBC
mode [RFC2451] MUST be supported and AES in Counter Mode [RFC3686]
SHOULD be supported. For use in high speed implementations, 3DES has
significant disadvantages. However, a 3DES IPsec transform has been
specified and parts are available that can run at 1 Gbps, so
implementing 3DES in products is practical for the short term.
As described in Appendix B, 3DES software implementations make
excessive computational demands at speeds of 100 Mbps or greater,
effectively ruling out software-only implementations. In addition,
3DES implementations require rekeying prior to exhaustion of the
current 32-bit IPsec sequence number space, and thus would not be
able to make use of sequence space extensions if they were available.
This means that 3DES will require very frequent rekeying at speeds of
10 Gbps or faster. Thus, 3DES is inconvenient for use at very high
speeds, as well as being impractical for implementation in software
at slower speeds (100+ Mbps).
5.6. Fragmentation Issues
When certificate authentication is used, IKE fragmentation can be
encountered. This can occur when certificate chains are used, or
even when exchanging a single certificate if the key size or size of
other certificate fields (such as the distinguished name and other
OIDs) is large enough. Many Network Address Translators (NATs) and
firewalls do not handle fragments properly, dropping them on inbound
and/or outbound.
Routers in the path will also frequently discard fragments after the
initial one, since they typically will not contain full IP headers
that can be compared against an Access List.
As a result, where IKE fragmentation occurs, the endpoints will often
be unable to establish an IPsec security association. The solution
to this problem is to install NAT, firewall or router code load that
can properly support fragments. If this cannot be done, then the
following alternatives can be considered:
[1] Obtaining certificates by other means.
[2] Reducing the size of the certificate chain.
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[3] Reducing the size of fields within the certificates. This
includes reduction in the key size, the distinguished name or
other fields. This should be considered only as a last resort.
Fragmentation can become a concern when prepending IPsec headers to a
frame. One mechanism that can be used to reduce this problem is to
utilize path MTU discovery. For example, when TCP is used as the
transport for iSCSI, iFCP or FCIP then path MTU discovery, described
in [RFC1191], [RFC1435], [RFC1981], can be used to enable the TCP
endpoints to discover the correct MTU, including effects due to IPsec
encapsulation.
However, Path MTU discovery fails when appropriate ICMP messages are
not received by the host. This occurs in IPsec implementations that
drop unauthenticated ICMP packets. This can result in blackholing in
naive TCP implementations, as described in [RFC2923]. Appropriate
TCP behavior is described in section 2.1 of [RFC2923]:
"TCP should notice that the connection is timing out. After
several timeouts, TCP should attempt to send smaller packets,
perhaps turning off the DF flag for each packet. If this
succeeds, it should continue to turn off PMTUD for the connection
for some reasonable period of time, after which it should probe
again to try to determine if the path has changed."
If an ICMP PMTU is received by an IPsec implementation that processes
unauthenticated ICMP packets, this value should be stored in the SA
as proposed in [RFC2401], and IPsec should also provide notification
of this event to TCP so that the new MTU value can be correctly
reflected.
5.7. Security Checks
When a connection is opened which requires security, IP block storage
security implementations may wish to check that the connection is
protected by IPsec. If security is desired and IPsec protection is
removed on a connection, it is reinstated before non-protected IP
block storage packets are sent. Since IPsec verifies that each
packet arrives on the correct SA, as long as it can be ensured that
IPsec protection is in place, then IP block storage implementations
can be assured that each received packet was sent by a trusted peer.
When used with IP block storage protocols, each TCP connection MUST
be protected by an IKE Phase 2 SA. Traffic from one or more than one
TCP connection may flow within each IPsec Phase 2 SA. IP block
storage security implementations need not verify that the IP
addresses and TCP port values in the packet match the socket
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information that was used to setup the connection. This check will
be performed by IPsec, preventing malicious peers from sending
commands on inappropriate Quick Mode SAs.
5.8. Authentication Issues5.8.1. Machine Versus User Certificates
The certificate credentials provided by the iSCSI initiator during
the IKE negotiation may be those of the machine or of the iSCSI
entity. When machine authentication is used, the machine certificate
is typically stored on the iSCSI initiator and target during an
enrollment process. When user certificates are used, the user
certificate can be stored either on the machine or on a smartcard.
For iFCP and FCIP, the certificate credentials provided will almost
always be those of the device and will be stored on the device.
Since the value of a machine certificate is inversely proportional to
the ease with which an attacker can obtain one under false pretenses,
it is advisable that the machine certificate enrollment process be
strictly controlled. For example, only administrators may have the
ability to enroll a machine with a machine certificate.
While smartcard certificate storage lessens the probability of
compromise of the private key, smartcards are not necessarily
desirable in all situations. For example, some organizations
deploying machine certificates use them so as to restrict use of
non-approved hardware. Since user authentication can be provided
within iSCSI login (keeping in mind the weaknesses described
earlier), support for machine authentication in IPsec makes it is
possible to authenticate both the machine as well as the user. Since
iFCP and FCIP have no equivalent of iSCSI Login, for these protocols
only the machine is authenticated.
In circumstances in which this dual assurance is considered valuable,
enabling movement of the machine certificate from one machine to
another, as would be possible if the machine certificate were stored
on a smart card, may be undesirable.
Similarly, when user certificate are deployed, it is advisable for
the user enrollment process to be strictly controlled. If for
example, a user password can be readily used to obtain a certificate
(either a temporary or a longer term one), then that certificate has
no more security value than the password. To limit the ability of an
attacker to obtain a user certificate from a stolen password, the
enrollment period can be limited, after which password access will be
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turned off. Such a policy will prevent an attacker obtaining the
password of an unused account from obtaining a user certificate once
the enrollment period has expired.
5.8.2. Pre-Shared Keys
Use of pre-shared keys in IKE Main Mode is vulnerable to man-in-the-
middle attacks when used with dynamically addressed hosts (such as
with iSCSI initiators). In Main Mode it is necessary for SKEYID_e to
be used prior to the receipt of the identification payload.
Therefore the selection of the pre-shared key may only be based on
information contained in the IP header. However, where dynamic IP
address assignment is typical, it is often not possible to identify
the required pre-shared key based on the IP address.
Thus when pre-shared key authentication is used in Main Mode along
with entities whose address is dynamically assigned, the same pre-
shared key is shared by a group and is no longer able to function as
an effective shared secret. In this situation, neither the initiator
nor Responder identifies itself during IKE Phase 1; it is only known
that both parties are a member of the group with knowledge of the
pre-shared key. This permits anyone with access to the group pre-
shared key to act as a man-in-the-middle. This vulnerability is
typically not of concern where IP addresses are typically statically
assigned (such as with iFCP and FCIP), since in this situation
individual pre-shared keys are possible within IKE Main Mode.
However, where IP addresses are dynamically assigned and Main Mode is
used along with pre-shared keys, the Responder is not authenticated
unless application-layer mutual authentication is performed (e.g.,
iSCSI login authentication). This enables an attacker in possession
of the group pre-shared key to masquerade as the Responder. In
addition to enabling the attacker to present false data, the attacker
would also be able to mount a dictionary attack on legacy
authentication methods such as CHAP [RFC1994], potentially
compromising many passwords at a time. This vulnerability is widely
present in existing IPsec implementations.
Group pre-shared keys are not required in Aggressive Mode since the
identity payload is sent earlier in the exchange, and therefore the
pre-shared key can be selected based on the identity. However, when
Aggressive Mode is used the user identity is exposed and this is
often considered undesirable.
Note that care needs to be taken with IKE Phase 1 Identity Payload
selection in order to enable mapping of identities to pre-shared keys
even with Aggressive Mode. Where the ID_IPV4_ADDR or ID_IPV6_ADDR
Identity Payloads are used and addresses are dynamically assigned,
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RFC 3723 Securing Block Storage Protocols over IP April 2004
mapping of identities to keys is not possible, so that group pre-
shared keys are still a practical necessity. As a result, identities
other than ID_IPV4_ADDR and ID_IPV6_ADDR (such as ID_FQDN or
ID_USER_FQDN) SHOULD be employed in situations where Aggressive mode
is utilized along with pre-shared keys and IP addresses are
dynamically assigned.
5.8.3. IKE and Application-Layer Authentication
In addition to IKE authentication, iSCSI implementations utilize
their own authentication methods. Currently, work is underway on
Fibre Channel security, so that a similar authentication process may
eventually also apply to iFCP and FCIP as well.
While iSCSI provides initial authentication, it does not provide
per-packet authentication, integrity or replay protection. This
implies that the identity verified in the iSCSI Login is not
subsequently verified on reception of each packet.
With IPsec, when the identity asserted in IKE is authenticated, the
resulting derived keys are used to provide per-packet authentication,
integrity and replay protection. As a result, the identity verified
in the IKE conversation is subsequently verified on reception of each
packet.
Let us assume that the identity claimed in iSCSI Login is a user
identity, while the identity claimed within IKE is a machine
identity. Since only the machine identity is verified on a per-
packet basis, there is no way for the recipient to verify that only
the user authenticated via iSCSI Login is using the IPsec SA.
In fact, IPsec implementations that only support machine
authentication typically have no way to distinguish between user
traffic within the kernel. As a result, where machine authentication
is used, once an IPsec SA is opened, another user on a multi-user
machine may be able to send traffic down the IPsec SA. This is true
for both transport mode and tunnel mode SAs.
To limit the potential vulnerability, IP block storage
implementations MUST do the following:
[1] Ensure that socket access is appropriately controlled. On a
multi-user operating system, this implies that sockets opened
for use by IP block storage protocols MUST be exclusive.
[2] In the case of iSCSI, implementations MUST also ensure that
application layer login credentials (such as iSCSI login
credentials) are protected from unauthorized access.
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If these directives are followed, then a rogue process will not be
able to access an IP block storage volume.
While the identity asserted within IKE is verified on a per-packet
basis, to avoid interference between users on a given machine,
operating system support is required. In order to segregate traffic
between users when user authentication is supported, the IPsec
endpoints must ensure that only traffic from that particular user is
sent or received within the IPsec SA. Enforcement of this
restriction is the responsibility of the operating system.
In kernel mode iSCSI drivers there typically is no user context to
perform user authentication. In this case the authentication is
closer to machine authentication. In most operating systems device
permissions would control the ability to read/write to the device
prior to mounting. Once the device is mounted, ACLs set by the
filesystem control access to the device. An administrator can access
the blocks on the device directly (for instance, by sending pass
through requests to the port driver directly such as in Windows NT).
In the same way, an administrator can open a raw socket and send
IPsec protected packets to an iSCSI target. The situation is
analogous, and in this respect no new vulnerability is created that
didn't previously exist. The Operating system's ACLs need to be
effective to protect a device (whether it is a SCSI device or an
iSCSI device).
5.8.4. IP Block Storage Authorization
IP block storage protocols can use a variety of mechanisms for
authorization. Where ID_FQDN is used as the Identity Payload, IP
block storage endpoints can be configured with a list of authorized
FQDNs. The configuration can occur manually, or automatically via
iSNS or the iSCSI MIB, defined in [AuthMIB].
Assuming that IPsec authentication is successful, this list of FQDNs
can be examined to determine authorization levels. Where certificate
authentication is used, it is possible to configure IP block storage
protocol endpoints with trusted roots. The trusted roots used with
IP block storage protocols might be different from the trusted roots
used for other purposes. If this is the case, then the burden of
negotiating use of the proper certificates lies with the IPsec
initiator.
Note that because IKE does not deal well with certificate chains, and
is typically configured with a limited set of trusted roots, it is
most appropriate for intra-domain usage.
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Since iSCSI supports Login authentication, it is also possible to use
the identities presented within the iSCSI Login for authorization
purposes.
In iFCP, basic access control properties stem from the requirement
that two communicating iFCP gateways be known to one or more iSNS
servers before they can engage in iFCP exchanges. The optional use
of discovery domains in iSNS yields access control schemas of greater
complexity.
5.9. Use of AES in Counter Mode
When utilizing AES modes, it may be necessary to use larger public
keys; the tradeoffs are described in [KeyLen]. Additional MODP
Diffie-Hellman groups for use with IKE are described in [RFC3526].
When AES in counter mode is used, it is important to avoid reuse of
the counter with the same key, even across all time. Counter mode
creates ciphertext by XORing generated key stream with plaintext. It
is fairly easy to recover the plaintext from two messages counter
mode encrypted under the same counter value, simply by XORing
together the two packets. The implication of this is that it is an
error to use IPsec manual keying with counter mode, except when the
implementation takes heroic measures to maintain state across
sessions. In any case, manual keying MUST NOT be used since it does
not provide the necessary rekeying support.
Another counter mode issue is it makes forgery of correct packets
trivial. Counter mode should therefore never be used without also
using data authentication.
6. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values of the SRP_GROUP
key parameter within iSCSI, in accordance with BCP 26, [RFC2434].
IANA considerations for the iSCSI protocol are described in
[RFC3720], Section 13; for the iFCP protocol in [iFCP], Section 12;
and for the FCIP protocol in [FCIP], Appendix B.
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RFC 3723 Securing Block Storage Protocols over IP April 20046.1. Definition of Terms
The following terms are used here with the meanings defined in BCP26: "name space", "assigned value", "registration".
The following policies are used here with the meanings defined in BCP26: "Private Use", "First Come First Served", "Expert Review",
"Specification Required", "IETF Consensus", "Standards Action".
6.2. Recommended Registration Policies
For registration requests where a Designated Expert should be
consulted, the responsible IESG Area Director should appoint the
Designated Expert.
For registration requests requiring Expert Review, the IPS mailing
list should be consulted, or if the IPS WG is disbanded, to a mailing
list designated by the IESG Area Director.
This document defines the following SRP_GROUP keys:
SRP-768, SRP-1024, SRP-1280, SRP-1536, SRP-2048, MODP-3072, MODP-
4096, MODP-6144, MODP-8192
New SRP_GROUP keys MUST conform to the iSCSI extension item-label
format described in [RFC3720] Section 13.5.4.
Registration of new SRP_GROUP keys is by Designated Expert with
Specification Required. The request is posted to the IPS WG mailing
list or its successor for comment and security review, and MUST
include a non-probabalistic proof of primality for the proposed SRP
group. After a period of one month as passed, the Designated Expert
will either approve or deny the registration request.
7. Normative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1191] Mogul, J. and S. Deering, "Path MTU Discovery", RFC1191, November 1990.
[RFC1435] Knowles, S., "IESG Advice from Experience with Path
MTU Discovery", RFC 1435, March 1993.
[RFC1981] McCann, J., Deering, S. and J. Mogul, "Path MTU
Discovery for IP version 6", RFC 1981, August 1996.
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Full Copyright Statement
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Aboba, et al. Standards Track [Page 70]